Astronomical Spectroscopy - Physics - University of Cincinnati

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So, attempt to select a telluric standard that has a similar declination to the target object,
but that transits 30-60 minutes before or after the target object.
Background emission is the second serious concern for the infrared spectroscopist. Even
optical astronomers are aware of the increase in night sky brightness with increasing wavelength,
with U and B brightness of typically >22 mag arcsec −2 , V around 21.5, R ∼ 21
and I ∼ 20 at the best sites. But this is nothing compared to what the infrared observer
must endure. A very nice review of infrared astronomy is given by Tokunaga (2000) that all
new (and seasoned!) infrared astronomers should read. He lists the sky brightness in mag
arcsec −2 as 15.9, 13.4 and 14.1 at J, H, and K s . For the L and M bands, the sky is 4.9 and
around 0 mag arcsec −2 , respectively! In the latter bands, this is dominated by thermal emission,
while in the J, H and K s , it is dominated by OH airglow. This background emission
will dominate one’s spectrum if not removed.
Removal of background emission is done by stepping the object along the slit between
exposures or periodically offsetting the telescope to a blank field. Since the background
emission is ubiquitous, offsetting a compact target along the slit allows one to measure the
background spectrum at the first target position in the second exposure, and vice versa. In
the near-infrared, where the sky background intensity is modest, one may step the target
to several positions in the slit, observing the target all of the time, while simultaneously
observing the sky background in the rest of the slit. However, if the field is densely populated
with stars, or the target itself is extended or surrounded by nebulosity, it is necessary to
offset to a nearby patch of blank sky periodically to obtain the sky spectrum for background
subtraction. At mid-infrared wavelengths, the sky background is significantly larger and
even small temporal variations can overwhelm the signal from the science target, so it is
necessary to carry out sky subtraction on a much shorter time scale. This is often done by
taking very short exposures (to avoid saturation) and chopping the target on and off the slit
at a few Hz, typically using a square-wave tip/tilt motion of the telescope secondary mirror.
The chopping and data taking sequences are synchronized, so that the on- and off-source
data can be stored in separate data buffers and the sky subtraction carried out in real time.
How often to step along the slit This depends on a few things. One always wants to
maximize the counts for any single step integration, letting the exposure time be determined
by the limits of the detector. Remember that you must stay within the linear regime while
including background emission in any single frame! If the integration is too long in the H-
band and the OH airglow lines are saturated, they won’t properly subtract. Always check
that the counts are not too high before any subtraction is done. The number of steps should
be at least four, to remove bad pixels and six is a more typical minimum number. Build
up signal-to-noise through multiple sets of optimized offsets, returning to a telluric standard